In the manufacture of disposable pens for various types of ink jet printers, various approaches have been taken to insure that a substantially constant backpressure (or sub-atmospheric pressure) is provided to the printhead of the pen as the ink is depleted from full to empty during an ink jet printing operation. In this manner, the size of the ink drops ejected from an orifice plate of the pen will remain constant during ink depletion, and additionally this backpressure will prevent leakage of ink from the orifice plate when an orifice is not firing. One such approach to providing a substantially constant backpressure in the ink reservoir of a thermal ink jet pen is disclosed and claimed in U.S. Pat. No. 4,509,062 issued to Robert Low et al and entitled "Ink Reservoir with Essentially Constant Negative Backpressure".
Whereas the approach described in the above Low et al patent has proven highly satisfactory and unique in most respects, this approach nevertheless requires and relies upon a collapsible bladder in order to maintain a substantially constant backpressure in the ink reservoir over a certain range of ink depletion therein. This requirement for a collapsible bladder has certain attendant disadvantages, in particular, volumetric inefficiency. These disadvantages have been overcome by the present invention and will be appreciated and better understood from the description to follow.
Another more recent approach to storing ink in an ink reservoir of a disposable ink jet pen and without using a collapsible bladder is disclosed and claimed in copending application Ser. No. 880,774 of Jeffrey Baker et al, filed July 1, 1986, assigned to the present assignee and incorporated herein by reference. In this latter approach, a reticulated polyurethane foam is used as an ink storage medium for both black and color pens. This more recent technique of storing the ink in a porous medium such as polyurethane foam poovides several new and useful improvements with respect to earlier bladder type storage techniques. However, the requirement for a porous storage medium in the main ink reservoir of the pen limits the volumetric storage efficiency of the ink reservoir. Also, the backpressure cannot be made as nearly constant ring ink depletion as in the method disclosed herein. And, the cut and cleaned foam adds a significant cost to the foam storage type pen.